The present invention concerns an additive for paper making, which is added to the fiber pulp prior to the web formation step in a paper making process. By means of the additive it is possible to impart to the paper i.a. a reduced tendency for dusting. In addition, the additive has been shown to facilitate water removal in the web formation stage, to improve filler retention, and to affect advantageously the removal of harmful substances, which have accumulated in the water circulation system of the paper machine. The additive also increases the strength of the finished paper, both dry strength and wet strength. By using the additive for paper making according to the invention improvements have been seen also in the printability of the paper, i.a. as regards its applicability for ink-jet printing.
A problem that is encountered in finished paper is its high tendency for dust formation, the reason for which are fiber particles and filler particles released from the paper surface. The dust gives rise to problems already at the drying stage in paper making, but above all in the machines and equipments handling paper, such as in printing equipment. The printing methods as such are developed which means high machine speeds and long printing series. High speeds aggrevate dust formation, and long printing series reduce standing times, during which it would be possible to carry out a cleaning of the equipments.
In order to solve the dusting problem it is known to use methods, which as a rule are based on chemical compounds to be added to the fiber pulp, prior to the paper web formation. The use of mineral and micro waxes, of sizing agents, such as AKD and ASA dispersions, of wet strength resins and of pulp sizing starch is known. Irrespective of these known measures, paper dusting is still a significant problem.
Now it has surprisingly been discovered that i.a. the dusting of paper can be reduced significantly by means of the additive according to the invention to be added to the fiber pulp prior to web formation.
The additive is based on starch, which has been modified to be applicable in the invention by reducing its molecular size and reacted with an appropriate nitrogen compound in order to provide a suitable cationic charge level to the starch.
The reduction of the molecular size has been carried out advantageously by oxidizing, such as by peroxide oxidation. The reduction of the molecular size is appropriately carried out so that the viscosity of a 5% starch suspension at 60xc2x0 C. is in the range 10-400 mPas (Brookfield). The viscosity is advantageously 100-400 mPas, and especially advantageously in the range 100-200 mPas. This is achieved for example using a hydrogen peroxide dose of approximately 0.02-0.3% from the starch dry matter in slightly alkaline reaction conditions. The desired degree of degradation is also bound to the desired cationic charge of the end product, since an increase in the cationic charge decreases the viscosity of the end product. There is also interdependence between the molecular size and the cationic charge which affects the behaviour of the starch in the paper machine.
Starch which has been processed to have the desired viscosity level is thereafter processed with a quaternary nitrogen compound according to the invention so that its cationic charge level will be in the range  less than 1.5, preferably in the range 0.36-1.46, especially advantageously in the range 0.72-1.10 mEqv/g, whereby the nitrogen contents when using,the quaternary cationizing chemical, will be in the range 0.5-2.5, correspondingly 1.0-1.5%. The product is advantageously prepared using solution cationization, in which the starch is fed into the cationization process in granular form, the process conditions are chosen so that the starch dissolves completely during the process. Essential process quantities in this respect are the concentration of the starch to be cationized, suitable alkalinity and increased temperature. The alkali dose (NaOH) is suitably in the range of approximately 1.5-3% of the starch dry matter, and the temperature suitably in the range of approximately 60-80xc2x0 C. The dry matter content of the reaction mixture should advantageously be over 50%, which gives i.a. a good yield for the end product. A suitable quaternary cationizing chemical is 2,3-epoxypropyltrimethylammonium chloride, which should be used in an amount of approximately 10-40% of the amount of starch.
The applicability of the invention is illustrated with the following examples, in which i.a. paper properties, which have an effect on the dusting of the paper in different paper handling conditions, have been monitored. Measuring the dusting tendency from a paper is as such problematic without a prolonged run of the paper in an application process, such as a printing operation. It is, however, generally known that the tendency for dust formation correlates to strength parameters which can be measured from the paper, such as Dennison, IGT and Scott Bond.